1. Field of the Invention
The present invention relates to hydraulic control valves for controlling flow of a fluid in an internal combustion engines, and more particularly to electrohydraulic valves for operating a mechanism which varies the phase relationship between a cam shaft and a crankshaft of the engine.
2. Description of the Related Art
Internal combustion engines have a plurality of cylinders containing pistons that are connected to a crankshaft. Each cylinder has two or more valves to control the flow of a fuel mixture into the cylinder and the flow of exhaust gases there from. Traditionally the valves were controlled by a cam shaft which in turn was mechanically connected to rotate with the rotation of the crankshaft. Gears, chains, or belts were used to couple the crankshaft to the cam shaft so that the two would rotate in unison. It is important that the valves open and close at the proper times during the combustion cycle within each cylinder. Heretofore, that timing relationship was fixed by the mechanical coupling between the crankshaft and the cam shaft.
The setting of the cam shaft timing often was a compromise which produced the best overall operation at all engine operating speeds. However, it was recognized that optimum engine performance could be obtained if the valve timing was varied as a function of engine speed, engine load and other factors. With the advent of computerized engine control, it became possible to determine the optimum engine valve timing based on the operating conditions occurring at any given point and time. With reference to FIG. 1, the engine computer determines the optimum valve timing and issues a signal to an electrohydraulic valve 10 which controls the flow of pressurized engine oil from a pump to a cam phase adjustment mechanism 12. The adjustment mechanism 12 couples the cam shaft 14 to a pulley or other mechanism that is connected to the engine crankshaft. By controlling the application of engine oil to either of two ports 18 or 19 of the adjustment mechanism, the phase relationship between the rotating pulley 16 and the cam shaft 14 can be varied. For example, application of engine oil from the pump to the first port 18 and exhausting engine oil from the second port 19 to the tank advances the valve timing. Whereas connecting the second port 19 of the adjustment mechanism 12 to the pump and coupling the first port 18 to the tank retards the valve timing. The hydraulic valve 10 is a proportional type valve which allows the amount that the cylinder valves are advanced or retarded to be proportionally varied by metering the flow of engine oil to and from the adjustment mechanism 12. A sensor 15 provides an electrical signal indicating the angular phase of the cam shaft.
Key to the operation of the variable cam shaft is the proper control of engine oil to the two port 18 and 19 and accurately metering that engine oil. Thus the control valve 10 becomes a critical element in the proper operation of the engine.
An electrohydraulic control valve includes a tubular valve body that has a longitudinal bore there through forming an outlet port at one end of the valve body. A first port, a second port and an inlet port extend transversely through the body and communicate with the longitudinal bore. A spool is slidably received within the bore of the valve body and has an aperture extending from an end of the spool that is proximate to the one end of the valve body to a point proximate an opposite end of the spool. The spool includes an notch in an exterior surface. A spring biases the spool away from the one end of the valve body.
An actuator comprises a solenoid coil wound on an annular bobbin with a tube of an electrically conductive, non-magnetic metal within the bobbin. A first pole piece of the actuator extends into one end of the tube and a second pole piece extends into another end of the tube. A first bushing is located in an aperture in the first pole piece and a second bushing is in another aperture in the second pole piece. Each of the first and second bushings has a tubular body with a first end section with a larger outer diameter than a second end section. The outer diameter of the first end section engages the respective pole piece. The second end section of each bushing has a smaller inner diameter than the first end section. The actuator also includes an armature is slidably received in the first and second bushings and engaging the spool.
The spool moves to several positions within the valve body depending upon the net force resulting from interaction of forces from the spring and the armature. In a first position the spool notch provides a first fluid path between the first port and the inlet port, and a second fluid path is provided between the second port and the outlet port. When the spool is at a second position, the notch provides a fourth fluid path between the inlet port and the second port and the aperture provides a fifth between the first port and the outlet port.
In an intermediate position of the spool, between the first and second positions, the outlet port is disconnected from the first and second ports. The notch can be manufactured to have one of several sizes to alter the connection provided in the intermediate position. A relatively short notch while being located adjacent to the inlet port does not extend to either the first or second ports. Therefore the first and second port are closed in the intermediate position. A relatively long notch forms a third fluid path that simultaneously connects the first port, the second port and the inlet port when the spool is in the intermediate position.